I have some of a Drupal book written, but I have frankly run out of steam on it. If you think the subject matter strikes your interest and you want to talk more, contact me. I started writing this for Drupal 4.7; much of it is done for Drupal 5, but Drupal 6 is in vogue now.
If you have a passion for writing or geeking out about Drupal, I'd love to hear from you. In a reversal of fate, I have an idea as to how to market the book (usually my conundrum).